linux-kernel - [PATCH v2 9/9] doc: Update documentation about list_for_each_entry

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Message-Id: <20190712170024.111093-10-joel@joelfernandes.org>
Date:   Fri, 12 Jul 2019 13:00:24 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Borislav Petkov <bp@...en8.de>, c0d1n61at3@...il.com,
        "David S. Miller" <davem@...emloft.net>, edumazet@...gle.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
        kernel-hardening@...ts.openwall.com, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
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        Oleg Nesterov <oleg@...hat.com>,
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        Pavel Machek <pavel@....cz>, peterz@...radead.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, will@...nel.org,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT))
Subject: [PATCH v2 9/9] doc: Update documentation about list_for_each_entry_rcu

This patch updates the documentation with information about
usage of lockdep with list_for_each_entry_rcu().

Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 Documentation/RCU/lockdep.txt   | 15 +++++++++++----
 Documentation/RCU/whatisRCU.txt |  9 ++++++++-
 2 files changed, 19 insertions(+), 5 deletions(-)

diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index da51d3068850..3d967df3a801 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -96,7 +96,14 @@ other flavors of rcu_dereference().  On the other hand, it is illegal
 to use rcu_dereference_protected() if either the RCU-protected pointer
 or the RCU-protected data that it points to can change concurrently.
 
-There are currently only "universal" versions of the rcu_assign_pointer()
-and RCU list-/tree-traversal primitives, which do not (yet) check for
-being in an RCU read-side critical section.  In the future, separate
-versions of these primitives might be created.
+Similar to rcu_dereference_protected, The RCU list and hlist traversal
+primitives also check for whether there are called from within a reader
+section. However, an optional lockdep expression can be passed to them as
+the last argument in case they are called under other non-RCU protection.
+
+For example, the workqueue for_each_pwq() macro is implemented as follows.
+It is safe to call for_each_pwq() outside a reader section but under protection
+of wq->mutex:
+#define for_each_pwq(pwq, wq)
+	list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node,
+				lock_is_held(&(wq->mutex).dep_map))
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 981651a8b65d..a08c03735963 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -290,7 +290,7 @@ rcu_dereference()
 	at any time, including immediately after the rcu_dereference().
 	And, again like rcu_assign_pointer(), rcu_dereference() is
 	typically used indirectly, via the _rcu list-manipulation
-	primitives, such as list_for_each_entry_rcu().
+	primitives, such as list_for_each_entry_rcu() [2].
 
 	[1] The variant rcu_dereference_protected() can be used outside
 	of an RCU read-side critical section as long as the usage is
@@ -305,6 +305,13 @@ rcu_dereference()
 	a lockdep splat is emitted.  See RCU/Design/Requirements/Requirements.html
 	and the API's code comments for more details and example usage.
 
+	[2] In case the list_for_each_entry_rcu() primitive is intended
+	to be used outside of an RCU reader section such as when
+	protected by a lock, then an additional lockdep expression can be
+	passed as the last argument to it so that RCU lockdep checking code
+	knows that the dereference of the list pointers are safe. If the
+	indicated protection is not provided, a lockdep splat is emitted.
+
 The following diagram shows how each API communicates among the
 reader, updater, and reclaimer.
 
-- 
2.22.0.510.g264f2c817a-goog